1. Field of the Invention
The present invention generally relates to a dual-lens hybrid diffractive/refractive imaging system and, more particularly, to an imaging system composed of a first lens and a second lens, in which an objective surface of the first lens is concave and any surface of the first lens and the second lens is spheric or aspheric. Moreover, the imaging system comprises at least one diffractive surface, referred to as a hybrid diffractive/refractive surface, formed on any surface of the two lenses. Therefore, aberration is eliminated and image quality is improved without increasing the number of lenses.
2. Description of the Prior Art
A conventional imaging system generally includes a plurality of refractive elements so as to achieve better image quality. For example, Taiwan Patent Publication No. 442685 entitled xe2x80x9cImaging lens systemxe2x80x9d discloses a imaging system comprising five lens elements, and Taiwan Patent Publication No. 432241 entitled xe2x80x9cHybrid lens systemxe2x80x9d discloses a imaging system comprising three lens elements. As a result, both the volume and the weight increase, which goes against the tendency towards size minimization of any consumer electronic product. However, with the rapid development in optics, there have been provided various diffractive elements, whereby multi-focusing, ring-shaped focusing and wavelength dividing can be achieved. Thus, by combining a diffractive element and a refractive element, better image quality is obtained without increasing the number of lenses.
Please refer to FIG. 1, which is a schematic diagram showing a prior art xe2x80x9cLENSES FOR ELECTRONIC IMAGING SYSTEMSxe2x80x9d disclosed in U.S. Pat. No. 6,097,551. The imaging system comprises a first lens 1 including a convex surface 11 and a concave surface 12, and a second lens 2 including a convex surface 21 and a concave surface 22. The convex surface 11 is an objective surface. The convex surface 21 facing the first lens 1 is a hybrid diffractive/refractive surface, while the concave surface 22 faces a charge-coupled device (CCD) 6. Even though better image quality can be obtained by using the convex surface 21 without increasing the number of lenses, the curvature of the first lens is large enough to make fabrication difficult. In addition, the convex surface 11 is an objective surface such that the field of view is limited, resulting in large size, large thickness, heavy weight and high cost of the first lens 1.
In view of the aforementioned issue, it is the primary object of the present invention to provide a dual-lens hybrid diffractive/refractive imaging system, characterized in that a diffractive element is profiled on a surface of a refractive lens such that aberration is eliminated and image quality is improved without increasing the number of lenses.
It is another object of the present invention to provide a dual-lens hybrid diffractive/refractive imaging system, in which a first lens has a small curvature that makes fabrication simple.
It is still another object of the present invention to provide a dual-lens hybrid diffractive/refractive imaging system, in which an objective surface of the imaging system is concave, resulting in a large field of view.
It is yet another object of the present invention to provide a dual-lens hybrid diffractive/refractive imaging system, in which the parameters used in the imaging system are easily obtained according to practical cases.